1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and an image forming system, which performs ground pattern printing to issue original prints.
2. Description of the Related Art
A printed material such as a certificate is demanded to be confirmable as an original. Hitherto, ground pattern printing is known as a technique that ensures the originality of a printed material. In the ground pattern printing, a document image is printed along with a ground pattern image corresponding to a ground pattern of characters or graphics that represent “issuer” or “certificate item”, for example. Although the ground pattern image is a latent image which can not be recognized even if the original printed material is visually seen, it can be recognized when a microscope, a color filter, or the like is used.
For example, JP-A-2007-104062 discloses an image processing apparatus for performing ground pattern printing. In this document, a ground pattern image includes a latent image part which is an image area where large dots are uniformly arranged at wide intervals, and a background part which is an image area where small dots are uniformly arranged at narrow intervals, and the average density of the latent image part and the average density of the background part are set to be almost equal to each other. By this, characters or graphics of a ground pattern become hard to be macroscopically recognized by the human visual sense, and is concealed in the original printed material. Further, the dot size of the latent image part is set to be larger than the minimum dot size as the limit of the reading and printing resolution of a general copier, and the dot size of the background part is set to be smaller than the minimum dot size. In the case where the original printed material is copied by a general copier, the dots of the latent image part remain in the duplicate similarly to a document image, and the dots of the background part disappear in the duplicate or becomes thinner than the dots of the latent image part. Accordingly, the dots of the latent image part become visible as the characters or graphics of the ground pattern in the duplicate. For example, when the characters of the ground pattern are “COPY”, it is easily understood from the revealed characters of the ground pattern that the duplicate is not the original.
Moreover, JP-A-2007-104062 proposes a ground pattern image in which characters of a ground pattern can be confirmed by an image processing using a scanner or the like. In this proposal, the image processing apparatus encodes (encrypts) “issuer” or “certificate item” as the characters or the graphics of the ground pattern by a secret key, generates a binary image pattern of a background part from the encoded information obtained as a result of this, generates a binary image pattern of a latent image part from a public key, and embeds ground pattern image data of a ground pattern image including these binary image patterns into document image data to perform the ground pattern printing. When the latent image part and the background part of the ground pattern image are read from the printed material obtained by this ground pattern printing, the public key is acquired from the binary image pattern of the latent part, the encoded information is acquired from the binary image pattern of the background part, and further, the encoded information is decoded by the public key. In the case where the characters or the graphics of the ground pattern are obtained as a result of the decoding, it becomes clear that this printed material is the original.
However, the image processing apparatus holds, for example, the characters or the graphics of the ground pattern, or an image pattern rule, as embedded electronic information necessary for generation of the ground pattern image data. Accordingly, there is a fear that falsification or analysis of the embedded electronic information is performed in the image processing apparatus, and a printed material such as a certificate is counterfeited or copied as the original.